Current transformer



Feb. 16, 1960 H. E. STROCK CURRENT TRANSFORMER Filed Aug. 12. 1954 INVENNR. H0 wmea faa'mws v 1. I u m y 7// v a 4 M k CURRENT TRANSFORMERHoward Eugene Struck, Charlotte, N.C., assignor to Howard Eugene Strockand Mason F. Leftwich, Charlotte, N.C., a partnership Application August12, 1954, Serial No. 449,430

4 Claims. (Cl. 336-96) My invention relates to a new and improvedcurrent or instrument transformer suitable for high voltage uses and themethod of making such a transformer. More specifically, it relates to amethod for providing a plastic housing for a transformer which will befree from stresses tending to crack or otherwise impair the efliciencyof such a housing, and for providing a single transformer suitable forboth indoor and outdoor use.

in the present method of constructing current or instrument potentialtransformers, it is the practice to surround the core and windings ofthe transformer with insulating material and then mount the insulatedtransformer in an aluminum or other type of metallic container. Theterminals for the transformer are brought out of the container bysuitable insulator bushings. The entire unit, including the container,must also be insulated.

This is the construction used for outdoor transformers. An indoortransformer consists simply of a core and protruding terminals with atape insulation winding of some nature on the coil.

When the latter type of transformer is modified for outdoor purposes,the insulation of the terminal from the container and of the containeritself presents a substantial problem and an element of considerableincrease in cost of the entire unit.

In my present invention I propose to provide a transformer suitable forboth indoor and outdoor use in which the core and windings are embeddedin a suitable thermosetting plastic material with terminals protrudingfrom the plastic. In this manner I obtain all of the required insulationand housing in one composition.

In embedding the transformer in a plastic encasing material, however,considerable difficulty has been encountered in the molding process,because of the necessary temperature variations occurring in such aprocess and the fact that the plastic has a much higher thermalcoefiicient of expansion and contraction than the metallic components ofthe transformer. This has resulted in excess shrinkage of the plasticupon cooling, which, in turn, sets up stresses and often cracks in theplastic casing. Thus in the molding operation a method must be found tocompensate for the excess shrinkage of the plastic. In this respectattempts at changing the formulation of the plastic have not provenparticularly successful or economical. Therefore, I also propose toeliminate or substantially reduce stresses or cracks which may occur inthe plastic casing material by providing a cushioning unit as anintegral part of the mold when the casting is made.

Another disadvantage of oil-filled or plastic-housed transformers of theprior art is apparent when the insulating medium is broken down by ahigh-voltage impulse. Often the breakdown occurs so quickly that gasesare exploded through the breakdown path. I propose to provide aconstruction which does not explode under such circumstances.

I further propose to provide as the encasing material a resin and fillerwhich will provide economies for the casting operation not heretoforerealized.

Accordingly, it is an object of my invention to provide an improvedcurrent transformer encased in a plastic material.

nited States Patent Silicone 2,925,570 Patented Feb. 16, 196i) It isanother object of my invention to provide an improved method forproducing a plastic-encased-transformer in which the casing is freefromstresses and cracks.

It is a further object of my invention to provide a cushioning meanswhich will relieve stresses resulting from the cooling of a plasticcasing material.

It is a still further object to utilize, in the casting of the plastic.encasing material, a cushioning material in the portion of thetransformer normally occupied by the primary winding which will allowthe plastic casing material as the coils to expand inwardly into saidspace.

Another object of my invention is to provide a special filler of highdielectric strength for the plastic material used to encase a highvoltage transformer.

A still further object of my invention is to provide an arrangement inthe plastic encasing material whereby flexible movement may be providedfor an insert placed in said encasing material.

Still another object of my invention is to provide an economical methodof casting a transformer in which no external heat is required in thecasting operation.

I Another object is to provide a transformer which will not explode whenbroken down by an electrical impulse high enough to exceed itsdielectric strength.

These and other objects of my invention will be more fully apparent fromthe detailed description of my invention when taken in connection withthe accompanying drawings, in which:

Figure 1 is an isometric and schematic view of the transformer as itappears in cross section during the casting operation.

Figure 2 represents a schematic view of the same transformer ready foruse and containing the primary conductor. I

Figure 3 represents a cross-sectional view of an outdoor-indoor currenttransformer made in accordance with the present invention.

Figure 4 represents an enlarged cross section of the arrangementprovided for placing an insert into the plastic casing material, e.g.insert 44 in Figure 3 turned upside down.

In accordance with my invention it has been found desirable to use asthe encasing material a plastic which can be molded without requiringexternal heat or electrically or steam-heated molds, thereby reducingthe cost of the final product.

I have found that these requirements are met by rigid, cross-linkedthermosetting resins of the polyester family such as those going underthe trade name of Vibrin or Permasil. The use of this type of resinmakes it possible to set core and coil of an electric transformer into amold and by proper formulation in mixing the polyester resin it can bepoured into the mold which would in turn set-up and cure withoutrequiring external heat.

In addition, I have found that the use of mica as a filler for thisresin has proven particularly advantageous because of its good strengthcharacteristics and dielectric qualities.

When making up my casing resin I have also found it desirable to includea polymerization promoter and a catalyst along with a substance whichwill decrease the moisture absorption of the final product.

A typical formulation is as follows:

Grams Vibrin 600 Calcium carbonate filler 60 Mica filler Promoter 12Catalyst 0.6 10

The polymerization promoter and catalyst may be any of a variety ofsubstances known to the art, and I have found that particularlysatisfactory results have been obtained with a promoter called LupricoJDB and a catalyst comprising a hydroxy cobalt napthenate dispersionmanufactured under the name of Cobalt Nuodex.

It should be understood, however, that the proportions of promoter andcatalyst can be regulated so that gelation and the final setting timecan be varied anywhere between minutes and 36 hours. It was found thatin the above formula the best results were obtained by havinga gelationtime of about 35 minutes at room temperature (72 F.)

The silicone is added to reduce the moisture absorption factor of thefinished "casting to a point where the plastic material is entirelysuitable for outdoor use or for submersion for long periods. Theparticular material selected for this function was silicone mixed in adispersing agent of styrene. It 'is also possible to reduce moistureabsorption by applying a silicone grease or dispersion to the outside ofthe casting.

The addition of the mica filler has been found to increase thedielectric strength of the plastic from 600 volts per mil thickness to1170 volts per miLbased on standard ASA test procedures.

The curing of the thermosetting polyester resin is exothermic. Thismeans that it is not necessary to apply outside heat in order for theresin to set. However, heat generation can be controlled by varying therelative amounts of catalyst and promoter. A substantial amount of heatgeneration is desirable in electrical castings in that it assists indrying out the core and coil and drying out any excess moisture whichmay happen to be in the core and coil. It was found that a temperaturein the range of 135 to 175 F. in the gelation and setting of the castingwas desirable.

In casting the transformer in a plastic it was found possible toeliminate the metal housing usually used in electric transformers and toeliminate the porcelain bushings, substituting the plastic material forthe porcelain insulator. Thus, this method of casting eliminatesconsiderable labor and material requirements and makes it possible tocast the entire insulator housing and molding terminals in oneoperation.

Various types of transformers may be cast in plastic.

These shapes include the unit type that is the casting in which core andcoil are embedded in one solid piece and the primary and secondary arebrought out through the insulaor as part of the casing; or the windowtype which has a round or square hole through the core of thetransformer to permit a single primary conductor to be passed throughthe core containing the secondary windings. This latter type oftransformer may also be adapted for high voltage in a manner wherein thesecondary leads are brought down through a pedestal type molding basewhich insulates the secondary windings from the primary. The pedestal isin the shape of an electrical outdoor type insulator but the casing isall in one piece with the housing. I

In my drawings and the following description I shall refer to this typeof transformer as my specific embodiment, but it should be clear thatthe principle of my novel casting method will be equally applicable toother types of transformers.

In casting a window type-transformer it has been found that whensetting, the plastic has a tendency to contract around the center hole.If, as in the case of an ordinary mold, this contraction is not allowed,stresses tend to be set'upi'n the plastic casing and in more severecases the casingh'as been known to crack. This occurs, particularly,"where the window comprises a square or. rectangular hole, therelativelysharp corners of which are particularly prone to stresses arising in thesetting operaof providing a cushion I have found that by coverirg theentire core and coils with a cushioning material such as corrugatedcardboard it is possible to allow the plastic to contract around thecore and coils without setting up undue stresses. More particularly itwas found that corrugated cardboard put on with masking tape was themost economical method to prevent stresses in the plastic material.

In order to eliminate stresses around the center, square hole which iscast into the transformer, the entire inside of the mold around thecenter hole is padded with corrugated paper and sealed with masking tapeto allow a cushion around the center hole and to permit contractionaround the hole to be relieved by this cushion.

The padding may be about /8 inch thick, for example, and sealed at alledges to preventthe plastic from running down behind and destroying thecushion effect.

The plastic material has an expansion or contraction rate OfZ /z timesthat of the steel and copper core and coil and the depth or size of thecushioning material can bedesigned in different units in a fashion toaccommo- 7 date this differential.

The mold may be filled in several ways: (1) Liquid plastic is pouredinto an opening in the bottom, top or sides of the mold. (2) Liquidplastic is drawn into the mold by means of a vacuum which to a greatdegree eliminates voids and air pockets.

Referring now to Figure 1, the transformer 30 illustrated here is of thewindow type with opening. 11 as the window which will eventually carrythe primary conductor. During the casting operation, a solid materialoccupies the space in order that a hole of the proper size may bemolded. Surrounding the inner and outer portions of secondary windingsof the core and coil of the secondary 1 2 is a layer of corrugated paperor cardboard 13 which acts as a cushion around the center hole 11 andthereby relieves the contraction of the plastic 14 in which the core andcoil 12 are embedded.

Figure 2 illustrates schematically the transformer as it will appearfrom the outside after the casting operation is completed. The core andcoil are all embedded in plastic housing 21 which carried a centeropening through which the primary conductor 22 has been passed.

Figure 3 shows in more detail a cross-section of this transformer takenalong the line 33 of Figure 2 and also includes the insulating baseportion of the transformer as it would appear where it is used as anindooroutdoor current transformer. This figure shows in more detailprimary conductor 31 as it passed through the core of the secondary 32and how the entire assembly is embedded in plastic 33. v

The pedestal type molded base insulates the secondary windings 34 fromthe primary 31. The pedestal here is in the shape of an electricaloutdoor type porcelain insulator with a corrugated surface 35 whichoffers increased resistance to crcepage and is cast in one piece withthe housing. Mounting bolts 36 are fastened to the core and perform thefunction of being reinforcing members and a grounding device to thecore, and can be bolted or strapped to the core as indicated at 3'7before or after winding. They provide the additional function offastening the terminal box 45 and mounting device to the pedestal 38.

The core is surrounded by fibre glass material which is incorporated inthe mold as a reinforcing material. This is shown by the dotted lines'39. This reinforcement prevents the casting from cracking fromdifferential expansion during the curing period. Instead of fibre glass,asbestos, cotton or linen may also be used but glass has been found tohave the best electrical characteristies and is therefore preferred. Thesecondary is also provided with concentric copper electrostatic shield40.

The insulated transformer is supported on a metallic base 41 withsecondary leads 42 and 43 extending downwardly to the base into insert44 of which Figure 4 is a detailed cross-section.

Referring now to Figure 4 the insert 50 is placed on the inside of themold 51 and held in place by means of screw 52. There is an over-sizedhole 53 in the molding 54 and a cushioning washer made of rubber or anyother flexible material 55 between the head of the screw 52 of the mold51.

Thus, although the insert 50 is firmly held in place it has bothlongitudinal and transverse movement due to the over-sized hole 53 andthe cushioning washer 55, respectively. The washer 55 then (1) providesa small amount of movement in the insert so that cracking will not occuraround the insert because of expansion and contraction of the plastic,(2) prevents leakage of the plastic outside the mold, and (3) holds theinsert in exact position.

It is to be understood that the transformer illustrated is illustrativeof my invention and that other inserts and other transformer designs mayalso be used in practicing my casting technique.

In the foregoing, I have described my invention only in connection withpreferred embodiment thereof. Many variations and modifications of theprinciples of my invention within the scope of the description hereinare obvious. Accordingly, I prefer to be bound not by the specificdisclosure herein but only by the appended claims.

I claim:

1. A transformer comprising a magnetic core and a secondary windingcarried on said core; said secondary winding including at least a firstand second lead conductor; said magnetic core and said secondary windingbeing embedded in a moulded plastic encasing member; said mouldedplastic encasing member having an extending pedestal portion forsupporting said transformer; said first and second lead conductors ofsaid secondary winding being embedded in said pedestal portion of saidmoulded plastic encasing member and extending to the exterior of saidpedestal.

2. A transformer comprising a magnetic core and a secondary windingcarried on said core; said secondary winding including at least a firstand second lead conductor; said magnetic core and said secondary windingbeing embedded in a moulded plastic encasing member; at least a portionof the surface of said instrument transformer having a relativelyflexible material interposed between its said surface portion and theadjacent surface portion of said plastic encasing member; said mouldedplastic encasing member having an extending pedestal portion forsupporting said transformer; said first and second lead conductors ofsaid secondary winding being embedded in said pedestal portion of saidmoulded plastic encasing member and extending to the exterior of saidpedestal.

3. A transformer comprising a magnetic core and a secondary windingcarried on said core; said secondary winding including at least a firstand second lead conductor; said magnetic core and said secondary windingbeing embedded in a moulded plastic encasing member; said mouldedplastic encasing member having an extending pedestal portion forsupporting said transformer; said first and second lead conductors ofsaid secondary winding being embedded in said pedestal portion of saidmoulded plastic encasing member and extending to the exterior of saidpedestal; said pedestal further including an embedded post meansextending from and secured to said core to an area external of thebottom of said pedestal; the external portions of said post meansforming mechanical connecting means for connecting said pedestal to asupport structure.

4. A transformer suitable for both indoor and outdoor use whichcomprises a magnetic core having a secondary winding thereon; saidtransformer being embedded in an integral moulded plastic encasingcomprising a polyester resin and a mica filler; the portions of saidintegral moulded plastic encasing toward said transformer embeddedtherein including a glass fibre material for reinforcing said integralmoulded plastic encasing during construction thereof and for increasingthe dielectric constant of said moulded plastic encasing.

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